Implement TWAP fallback for Oracle price ingestion

[nanaf6203-bit]

Problem Statement

Median calc fails or sources stall.

Why it matters

Resilience.

Technical Context

Add historical rolling TWAP; fallback when median is undefined.

Expected Outcome

Stable fallback.

Acceptance Criteria

1. Tests cover stale median.
2. Kani harness.
3. SDK.
4. Docs.

Implementation Notes

New module oracle::twap.

Files or modules likely to be affected

contracts/oracle/src/twap.rs.

Dependencies

None.

Difficulty level

HARD.

Estimated effort

L (~1 week).